1. Field of the Invention
The present invention relates to a driving force control device that controls a gear ratio of an automatic transmission to achieve a target driving force or a target driving torque (hereinafter only referred to as a target driving force) which is calculated by a vehicle distance control unit, a vehicle speed control device and so forth.
2. Description of the Prior Art
Conventionally, Japanese Patent Publication No. 06-68324 is known as an example of a conventional control device for an automatic transmission in a vehicle that is equipped with the automatic transmission and an automatic speed control device which maintains a constant vehicle speed.
According to the publication, the object is to properly control a shifting operation while a constant-speed regulation is executed and also control the shifting operation correspondingly to a driver""s operation while the constant-speed regulation is not executed. According to the publication, the shifting operation is controlled according to a shift map based on a throttle opening (angle) of a throttle valve while the constant-speed regulation is executed, and the shifting operation is controlled according to a shift map based on an operated input of an accelerator pedal while the constant-speed regulation is not executed.
According to the above conventional control device for the automatic transmission, however, the two shift maps must be prepared to control the shifting operation. This necessitates setting the shift transition characteristics for the respective shift maps, and makes it complicated to apply the control device to the vehicle.
More specifically, if the automatic transmission is a multi-speed transmission, it is necessary to reduce a shift shock by controlling hydraulic pressure and line pressure in a shift transitional period in different manners between the two shift maps. If the automatic transmission is a continuously variable transmission, it is necessary to properly tune a shift speed by controlling the shift transition in different manners between the two shift maps.
There is the following disadvantage if the conventional control device for the automatic transmission is applied to a driving force control device that achieves a target driving force by changing a gear ratio of the automatic transmission in a vehicle equipped with a device for controlling a vehicle distance (the distance between the vehicle and a vehicle running ahead) or a device for controlling a vehicle speed by the driving force. Because the different shift maps are used according to whether the driving force is controlled or not, the shift transition characteristics must be set for the respective shift maps. It is therefore complicated to apply the control device to the vehicle.
It is therefore an object of the present invention to provide a driving force control device that is able to control a driving force by changing a gear ratio of an automatic transmission with the same shift transition characteristic irrespective of whether the driving force is controlled or not.
The above object can be accomplished by providing a driving force control device, which controls a gear ratio of an automatic transmission to thereby achieve a target driving force calculated by an onboard control device, the driving force control device comprising: vehicle speed sensor for detecting a vehicle speed; throttle opening sensor for detecting a throttle opening; virtual throttle opening converter for calculating a driving force that is outputted from the automatic transmission according to a predetermined shift map based on the vehicle speed and the throttle opening, setting a target driving force characteristic whose parameters are the vehicle speed and the throttle opening according to a result of the calculation, and converting the target driving force into a virtual throttle opening by using the target driving force characteristic and the detected vehicle speed; throttle opening information selector for selecting the virtual throttle opening from the virtual throttle opening converter as throttle opening information when a driving force is controlled, and selecting an actual throttle opening from the throttle opening sensor as throttle opening information when a driving force is not controlled; and target shift value calculating means for calculating a target shift value according to the selected throttle opening information, the detected vehicle speed value and said predetermined shift map.
In one preferred form of the present invention, the onboard control device calculates a target driving force so as to maintain a constant vehicle distance or vehicle speed.
In another preferred form of the present invention, the virtual throttle opening converter outputs the converted virtual throttle opening to a gear ratio control system of the automatic transmission and a throttle control system of an engine so as to control an engine torque and a gear ratio of the automatic transmission.
In yet another preferred form of the present invention, the automatic transmission is a continuously variable transmission that determines an achievable gear ratio, which should finally be achieved, according to a target input revolutionary speed calculated by the target shift value calculating means and a detected output revolutionary speed, and a transitional target gear ratio until the achievable gear ratio is reached, thereby controlling the gear ratio continuously.
In yet another preferred form of the present invention: the virtual throttle opening converter makes the target driving force dimensionless by using a maximum driving force for an applied vehicle speed and then converts the target driving force into the virtual throttle opening.
According to the present invention, the onboard control device calculates the target driving force; and its virtual throttle opening converting means calculates a driving force that is outputted from the automatic transmission according to a predetermined shift map based on the vehicle speed and the throttle opening, sets the target driving force characteristic whose parameters are the vehicle speed and the throttle opening based on the result of the calculation, and converts the target driving force into the virtual throttle opening by using the target driving force characteristic and the detected vehicle speed. The throttle opening information selecting means selects the virtual throttle opening from the virtual throttle opening converting means as throttle opening information when the driving force is controlled, and selects the actual throttle opening from the throttle opening sensing means as throttle opening information when the driving force is not controlled. The target shift value calculating means calculates a target shift value according to the selected throttle opening information, the detected vehicle speed value and the predetermined shift map. The gear ratio of the transmission is controlled in such a manner as to achieve the target shift value. The target driving force can be achieved by controlling the output from the transmission in this manner.
More specifically, the target shift value calculating means calculates the target shift value by using the throttle opening information selected between the virtual throttle opening and the actual throttle opening. Therefore, only one shift map should be set irrespective of whether the virtual throttle opening is outputted or not i.e. whether the driving force is controlled or not.
It is therefore possible to control the driving force by changing the gear ratio of the automatic transmission with the same shift transition characteristic irrespective of whether the driving force is controlled or not. Thus, the load of matching constants is reduced, and a computer program is simplified. This makes it easier to apply the driving force control device to the vehicle.
When the control device provided in the vehicle calculates the target driving force so as to maintain a constant vehicle distance or a constant vehicle speed, it is possible to control the driving force in such a manner as to maintain a constant vehicle distance or vehicle speed by changing the gear ratio of the automatic transmission without adding a new shift map to the shift map that is used in the normal shift control.
In a case where the virtual throttle opening converting means outputs the converted virtual throttle opening to the gear ratio control system of the automatic transmission and the throttle control system of the engine, the driving force is controlled by controlling both the engine torque and the gear ratio of the automatic transmission. This makes it possible to achieve the target driving force with excellent responsiveness and preferable convergence with respect to changes in the target driving force.
When the automatic transmission is a continuously variable transmission that determines an achievable gear ratio, which should finally be achieved, according to the target input revolutionary speed calculated by the target shift value calculating means and the detected output revolutionary speed, and the transitional target gear ratio until the achievable gear ratio is reached, thereby controlling the gear ratio continuously, the driving force is controlled by continuously changing the gear ratio so that the driving force can correspond to the target driving force. Moreover, the transition target gear ratio is determined according to the achievable gear ratio, and thus the shift transition characteristic is stable since it is affected by the changes in the shift environment to only a small extent.
To the contrary, if, for example, the automatic transmission is a multi-speed transmission, the change in the shift position causes the gear ratio to change greatly. It is therefore impossible to control the driving force in such a manner that the driving force strictly corresponds to the target driving force. Moreover, the shift transition characteristic is unstable because it is greatly affected by the change in the shift environment.
In the present invention, if the virtual throttle opening converting means makes the target driving force dimensionless by using the maximum driving force for an applied vehicle speed and then converts the target driving force into the virtual throttle opening, the sensitivity per vehicle speed of the driving force-throttle opening is leveled by making the target driving force dimensionless by using the maximum driving force for the applied vehicle speed, and this improves the accuracy in calculating the virtual throttle opening.